Atmospheric laser propagation

Photonics SpectraMar 2011
An underwater photo-ionization laser acoustic source developed by researchers
at the US Naval Research Laboratory could enable remote acoustic generation from
compact air- and shipborne lasers, allowing aircraft to communicate without the
need for source hardware in the water.

The investigators successfully completed long-distance acoustic
propagation and shock-generation field testing of the device at the Lake Glendora
Test Facility of Naval Surface Warfare Center located in Crane, Ind.

Housed in a floating structure, the Nd:YAG laser
(bottom left) generates underwater acoustic pulses that travel to a distant hydrophone-equipped
boat (bottom right). Courtesy of US Naval Research Laboratory.
Using a pulsed Nd:YAG 532-nm laser housed on a floating platform,
they directed pulses by steering mirrors through a focusing lens and into the water
surface. Each laser pulse produced acoustic pulses with sound pressure levels measuring
~190 dB. These were detected by boat-mounted hydrophones at distances of up to 140
m. Previous lab acoustic propagation distances were limited to ~3 m.

Because the laser pulses can travel through both air and water,
a compact laser on either an underwater or airborne platform can be used for remote
acoustic generation. Properly tailored pulses can travel many hundreds of meters
through the air, remaining relatively unchanged, then quickly compress upon entry
into water.

With the potential to expand and improve both naval and commercial
underwater acoustic applications, the sources could be efficient tools for undersea
communications from aircraft.